The invention relates to a digital system including a coder and a decoder for subband coding of a digital audio signal having a given sampling rate 1/T, the coder comprising:
analysis filter means responsive to the audio signal, for generating a number of P subband signals, the analysis filter means dividing the audio signal band according to the quadrature mirror filter technique, with sampling rate reductin, into successive subbands of band numbers p (1.ltoreq.p.ltoreq.P) increasing with the frequency, the bandwidth and the sampling rate for each subband being an integral submultiple of 1/(2T) and 1/T, respectively, and the bandwidths of the subbands approximately corresponding with the critical bandwidths of the human auditory system in the respective frequency ranges, PA1 means responsive to each of the subband signals, for determining a characteristic parameter G(p;m) which is representive of the signal level in a block having a same number of M signal samples for each subband, m being the number of the block, PA1 means for adaptively quantizing the blocks of the respective subband signals in response to the respective characteristic parameters G(p;m); PA1 means for adaptively dequantizing the blocks of the quantized subband signals in response to the respective characteristic parameters G(p;m), PA1 synthesis filter means responsive to the dequantized subband signals for constructing a replica of the digital audio signal, these synthesis filter means merging the subbands to the audio signal band according to the quadrature mirror filter technique, with sampling rate increase. PA1 the respective quantizing means in the coder and the respective dequantizing means in the decoder for each of the subbands having a band number p smaller than p.sub.im are arranged for the respective quantizing and dequantizing of the subband signals with a fixed number of B(p) bits, the subband having band number p.sub.im being situated in the portion of the audio signal band with the lowest thresholds for masking noise in critical bands of the human auditory system by single tones in the centre of the respective critical bands, PA1 the coder and the decoder each further include bit allocation means responsive to the respective characteristic parameters G(p;m) of the subbands having a band number p not smaller than p.sub.im within an allocation window having a duration equal to the block length for the subband having the band number p.sub.im, for allocating a number of B(p;m) bits from a predetermined fixed total number of B bits for the allocation window to the respective quantizing means in the coder and the respective dequantizing means in the decoder for the signal block having block number m of the subband having band number p, the bit allocation means each comprising: PA1 do not allocate any quantizing bits to block (p;m+1) and add the numbers of B(p;m+1) quantizing bits available for this block to the said sum S, if the ratio Q=G(p;m)/G(p;m+1) ratio is greater than a predetermined value R(p) of the order of 10.sup.2 and block (p;m+1) is situated within the allocation window; PA1 do not allocate any quantizing bits to block (p;m) and add the numbers of B(p;m) quantizing bits available for this block to the said sum S, if the ratio Q=G(p;m)/G(p;m+1) is smaller than the value 1/R(p) and block (p;m) is situated within the allocation window.
and the decoder comprising:
A system for subband coding of a similar structure is known from the article entitled "The Critical Band Coder--Digital Encoding of Speech Signals Based on the Perceptual Requirements of the Auditory System" by M. E. Krasner, published in Proc. IEEE ICASSP 80, Vol. 1, pp. 327-331, Apr. 9-11, 1980.
In this known system, use is made of a subdivision of the speech signal band into a number of subbands, whose bandwidths approximately correspond with the bandwidths of the critical bands of the human auditory system in the respective frequency ranges (compare FIG. 2 in the article by Krasner). This subdivision has been chosen because on the basis of psychoacoustic experiments it may be expected that in a suchlike subband the quantizing noise will be optimally masked by the signals within this subband when the quantizing takes account of the noise-masking curve of the human auditory system (this curve indicates the threshold for masking the noise in a critical band by a single tone in the centre of the critical band, compare FIG. 3 in the article by Krasner).
In the case of a high-quality digital music signal, represented according to the Compact Disc standard with 16 bits per signal sampling at a sample rate of 1/T=44.1 kHz, it appears that the use of this known subband coding with a suitably chosen bandwidth and a suitably chosen quantizing for the respective subbands results in quantized output signals of the coder which can be represented with an average number of 2.5 bits per signal sample, while the quality of the replica of the music signal does not perceptibly differ from that of the original music signal in virtually all passages of nearly all sorts of music signals. However, in certain passages of some sorts of music signals the quantizing noise is still audible. The audibility of the quantizing noise can be reduced by increasing the number of quantizing levels, but this implies that the average number of bits per sample of the quantized output signals of the coder then has to be increased too.